Reference is made to U.S. Pat. No. 4,731,044, to Philip J. Mott, assigned to Borg-Warner Automotive, Inc., which is incorporated herein by reference and upon which the present invention is an improvement.
1. Field of the Invention
The present invention relates to continuously variable transmissions and control systems for continuously variable transmissions. More particularly, the present invention relates to a control strategy for continuously variable transmissions which utilizes the relationship between force ratio, speed ratio and the factor of safety to determine the output force necessary for the secondary sheave to prevent belt slippage.
2. Description of the Prior Art
Variable pulley transmissions, or continuously variable transmissions (CVTs), known in the prior art primarily for automotive applications, typically comprise a pair of pulleys with an adjustable diameter, an endless belt connecting the pulleys, and a control unit. In conventional continuously variable transmissions, a primary variable pulley or primary sheave system is provided on a primary or input shaft and a secondary variable pulley or secondary sheave system is provided on the secondary or output shaft. Each pulley has at least one sheave that is axially fixed and another sheave that is axially moveable relative to the first sheave. The inner faces of the sheaves are bevelled so that as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and, thus, the effective pulley diameter may be adjusted. Adjustment of the primary and secondary pulleys thus varies the ratio between the input and output shafts. The primary pulley is conventionally driven by the engine and the output of the secondary pulley drives the drive train of the vehicle. U.S. Pat. No. 4,433,594, titled "Variable Pulley Transmission," provides further information in relation to CVT's and is incorporated herein by reference in terms of background to the present invention.
In conventional continuously variable transmission's the primary sheave is utilized to set the speed ratio, or ratio between the speed of rotation of the input and output shafts. Depending on the desired output speed ratio, the variable pulley on the input shaft is moved axially to adjust the position of the belt within the pulley. The volume of fluid inputted to the servo mechanism of the primary sheave determines the location of the adjustable sheave of the variable pulley along the shaft. Such movement sets the ratio between the input and output shafts.
Once the speed ratio is set by adjustment of the primary sheave, the secondary sheave is utilized to clamp the belt and prevent the belt from slipping. The control unit sends electrical signals to the secondary servo mechanism to provide pressure based on a number of inputs to the control unit. Those inputs include engine output speed, transmission output speed, throttle position and gearshift position. Utilizing this information, the microprocessor in the control unit sets desired values for engine speed, CVT system pressure and clutch coolant. The control system uses these signals to regulate the CVT ratio and the CVT line pressure and maintain the requisite belt tension.
The volume of fluid inputted to the servo mechanism of the secondary sheave, which determines the force of the sheave on the belt to maintain belt tension, is typically determined by a several step procedure. First, the position of the throttle of the vehicle is sensed and a calculation of the engine output torque is made using the torque map in the control system microprocessor. Based on the determined torque and an assumed coefficient of friction for the belt, a determination of the necessary force on the belt to prevent slipping is made by the microprocessor. An appropriate volume of fluid is then sent to the servo mechanism of the secondary sheave to press the sheave against the belt.
The "factor of safety" is a calculation of the amount of actual force on the belt generated by the secondary sheave divided by the amount of force on the belt by the secondary sheave necessary to keep the belt from slipping. The conventional continuously variable transmission control system described above results in an excessive factor of safety on account of the imprecise nature of the system. The system is necessarily imprecise as a result of the reliance on an estimation of the amount of engine torque from the torque map.
An excessive factor of safety results in excess force on the secondary sheave and therefore causes inefficiencies in the system. For example, the excessive force on the secondary to clamp the belt can cause losses in the efficiency of the system by excess friction and can also result in belt wear and decreased automobile fuel economy.
The control strategy of the present invention relies on a relationship between the force ratio, the speed ratio and the factor of safety to determine the necessary force on the secondary sheave. Such a system improves the efficiencies associated with the belt and pulley system of the continuously variable transmission.